The process of manufacturing integrated circuits typically consists of more than a hundred steps during which hundreds of integrated circuits are formed on a single semiconductor wafer. Generally, this process involves the creation of multiple layers on the semiconductor wafer This layering process forms the electrically active regions on the wafer's surface.
After completion of each layer, it is common practice to thoroughly clean, rinse, and dry the wafer. Because the formation of each layer depends on the accuracy and precision with which the previous layer was formed, it is critical that each layer be free of contaminants or surface impurities. In recent years, the smaller size, and hence increased density of microelectronic components, has increased the demand for more effective control of contamination.
A current method for removing contaminants from the surface of a semiconductor wafer uses a sponge roller to mechanically scrub the wafer's surface in the presence of a cleaning solution. In some practices of this method, cleaning solution is applied either by directing a high pressure spray against the wafer or by dripping cleaning solution on the sponge rollers. Typically, the wafer is positioned horizontally between two rotating sponge rollers. During this scrubbing cycle, the wafer is rotated by an edge handling apparatus to ensure that the sponge rollers engage and scrub substantially the entire surface of the wafer.
Upon completion of the scrubbing cycle, the wafer is typically transferred to a rinsing-drying stage separate from the scrubbing stage. Typically, this transfer is effected by moving conveyor belts on which the wafer rests during the scrubbing cycle. These belts, which frictionally engage the bottom surface of the horizontally disposed wafer, are prone to leave residue on the wafer.
In this rinsing-drying stage, the wafer typically rests horizontally on a spindle chuck which spins at high speeds. A stream of de-ionized water sprayed onto the surface of the spinning wafer forms a thin film on the wafer's surface. In some cases, an acoustic transducer mounted on a probe hydroplaning on the surface of this film generates megasonic waves that propagate through the film. These waves assist in dislodging any particles remaining after the scrubbing cycle. As the wafer spins, the de-ionized water, and any contaminant particles suspended within it, are driven off the periphery of the wafer by centrifugal force. This eventually dries the wafer surface.
Commonly used wafer diameters have thus far been small enough so that the internal forces generated during the spinning of the wafer are, in most cases, insufficient to stress or break the wafer. However, the recently introduced 300 mm wafers are so large that internal forces generated during the spinning of the wafer significantly increase the probability of breakage. This is further exacerbated by the fact that, in 300 mm wafers, the de-ionized water travels further to reach the periphery. Consequently, spin drying 300 mm wafers requires high rotational speed to generate enough centrifugal force to rapidly move the de-ionized water from the center to the periphery of the wafer.
The foregoing wafer cleaning method has additional disadvantages which are unrelated to the size of the wafer. For example, megasonic energy is incident on the wafer only during the rinsing step. The lack of megasonic energy during the scrubbing step forecloses any synergistic operation between the operations of brush scrubbing and of megasonic bombardment. In addition, the sponge rollers, which are never exposed to the cleaning effect of megasonic energy, are prone to become dirty and to wear prematurely. This significantly increases the operating costs of the wafer-scrubbing machine.
Because megasonic energy effective for cleaning generally requires transmission through a liquid, devices for megasonic cleaning provide for submerging a wafer in a liquid while exposing it to megasonic energy. It has long been considered, therefore, that an apparatus for simultaneously scrubbing a substrate while it is exposed to megasonic energy would require immersion of moving mechanical parts. Any stray particles generated by these moving mechanical parts would contaminate the surrounding liquid. As a result, prior art cleaning devices typically separate the brushing step from the megasonic cleaning step, as in the manner set forth above. Consequently, these cleaning devices forego any synergistic effect associated with simultaneous brushing and megasonic cleaning.
In the foregoing prior method, a cleaning solution consisting of one or more liquids is typically mixed together and applied to the surface of the wafer, either by spraying the mixture onto the surface of the wafer or by passing it through a sponge roller mounted on a hollow brush core and allowing it to perfuse through the sponge roller. However, certain mixtures lose their cleaning potency soon after they are mixed. It is preferable that such cleaning mixtures be applied to the surface shortly after they are mixed.
Since a typical cleaning cycle can use several different cleaning solutions at different stages of the cycle, it is advantageous to provide a system for quickly and efficiently switching from a first cleaning solution to a second cleaning solution. In the case in which the cleaning solution is passed into a hollow brush core for perfusion through the sponge, an appreciable volume of cleaning solution remains in the brush core. Thus, in order to change cleaning solutions, a significant residual volume of the first cleaning fluid must be removed from the brush core before the second cleaning fluid can be used. In cases in which there exists an undesirable reaction between the old and new cleaning solutions, this already time-consuming step is further lengthened by the need to flush the first cleaning solution with a non-reactive fluid rather than with the second cleaning solution.
The known practice of exposing the wafer to the second cleaning solution by transporting it to a different scrubbing stage as an alternative to changing the cleaning fluid in a scrubbing stage is unsatisfactory. This known practice requires additional clean room space to accommodate the additional scrubbing stage. In addition, the wafer transport mechanism for transporting the wafer from one scrubbing stage to another can introduce contaminants.
In the known method for cleaning a wafer with a cylindrical brush-sponge, it is preferable to rotate the wafer as it is being scrubbed to insure that all points on the surface of the wafer are exposed to the sponge roller. Current methods of rotating the wafer include the use of rim driving wheels that rotate the wafer by engaging its periphery. These wheels, which are typically made of a hard plastic such as polyurethane, can undermine the entire cleaning process by leaving contaminants on the peripheral region of the wafer.
The prior art includes numerous attempts to avoid the foregoing disadvantages. For example, in some wafer cleaning machines, a nozzle sprays liquid on the periphery of the wafer as the wafer is rotated in an effort to remove particles left behind by the rim driving wheels. Other wafer cleaning machines provide rim driving wheels made of relatively soft plastic materials.
The pressure exerted on the wafer by the sponge roller is an important factor in effective cleaning of wafers. If this sponge pressure is too low, scrubbing is largely ineffective. If the sponge pressure is too high, particulates can inflict damage on the wafer by gouging its surface. The applied sponge pressure is determined, in part, by the thickness of the sponge and by the thickness of the wafer. If each thickness is known, one can readily apply a selected pressure by setting an appropriate distance between the sponge and the wafer.
The difficulty encountered in the foregoing method of controlling brush pressure is that these thicknesses are dynamically changing quantities. For example, as the sponge roller wears, its thickness can decrease. This typically results in a reduction in the brush pressure and an accompanying reduction in cleaning effectiveness. It is known in the art to automatically adjust the distance between the sponge roller and the wafer according to a predetermined schedule based on empirical observations of the sponge roller's wear as a function of time. However, such a method, relying as it does on a statistically derived quantity, can easily result in application of pressure that is not the optimal pressure.
When the sponge roller ultimately wears out, it becomes necessary to replace it. In current wafer cleaning machines, replacement is accomplished by removing the worn sponge roller from the brush core and inserting the brush core through a fresh sponge roller. Since the sponge roller is typically a cylindrical tube of soft sponge-like material having an inner diameter slightly smaller than the outer diameter of the brush core, it is difficult and time consuming to insert the brush core through the sponge roller. The difficulty of this task frequently results in damage to or contamination of the sponge roller. This results in increased operational expenses, because damaged sponge rollers must be discarded, or reduced yield, because contaminated sponge rollers are unlikely to clean wafers effectively.